Dissolution of platinum in methoxy propionitrile containing LiI/I2

The stability of vapour deposited Pt and Pd as electrocatalysts on fluorine doped, tin oxide coated glass for the use as counter electrodes in dye senzitized electrochemical solar cells has been investigated. The electrocatalytically active layers did not seem to be chemically stable in an electrolyte consisting of LiI and I₂ dissolved in methoxy propionitrile. Thermodynamical calculations suggest the dissolution of Pt and Pd from the electrode surface to be caused by the formation of PtI₄ and PdI₆. Formation of complex species as PtI²⁻₄ and PdI²⁻₄ is less thermodynamically favoured, but PtI²⁻₄ may also be present in the solution.     

Investigation of influence of redox species on the interfacial energetics of a dye-sensitized nanoporous TiO2 solar cell

The photoelectrochemical behaviours of dye-sensitized nanoporous TiO₂ solar cells are studied under influences of light intensity, redox couple concentration, temperature, different cations and water in the nonaqueous solution. The value of the ideality factor of dyed nanoporous TiO₂ film is determined to be 1.08. The diode behaviour of the dyed nanoporous TiO₂ film approaches an ideal rectification characteristic. The rate of the reaction of I₃⁻ with the electron at the surface of the dyed TiO₂ electrode is of first order, like the reduction of I₃⁻ at the Pt electrode. By analysis of the relationship of the photovoltage with temperature, the activation energies of the back-reaction for dyed nanoporous TiO₂ electrodes in different solutions are obtained. Cations of different kinds and water are found to modify the interfacial properties of the dyed TiO₂ electrode. Finally, a quantitative relationship between the short-circuit photocurrent and the light intensity, the I₃⁻ concentration is obtained and used to explain the diffusion-controlled photocurrent. The corrected diffusion coefficient of I₃⁻ is 5.4–6.2×10⁻⁶ cm²/s in a CH₃OCH₂CN solution.     

Photoelectrochemical cells consisting of phenosafranin-EDTA and different redox couples with illuminated semiconductor electrode

A photoelectrochemical cell, semiconductor (In₂O₃)/dye-EDTA//redox couple/Pt, has been developed using phenosafranin dye and EDTA aqueous solution in one compartment and Cu⁺/Cu²⁺, Fe(CN)₆⁴⁻/Fe(CN)₆³⁻, I⁻/I₂, and Fe²⁺/Fe³⁺ in the other compartment of an H-shaped cell separated by a glass membrane. All the cell characteristics such as open-circuit voltage, short-circuit current, fill factor, power efficiency, and solar energy efficiency have been determined. There is 2-3-fold increase of efficiency of the cell compared to the same cell with illuminated Pt electrode.     

Influence of alkyl dihalide gelators on solidification of dye-sensitized solar cells

Quasi-dye-sensitized solar cells were prepared by using ionic liquid-type electrolytes and gelators consisting of polyvinylpyridine and alkyl dihalides. Gelation occurred by the reaction of polyvinylpyridine and alkyl dihalides. When the chain length of the dihalides was varied, the short-circuit current (J_sc) increased with an increase in the chain length. However, the open-circuit voltage (V_oc) and fill factor (ff) slightly decreased. The increase in J_sc was brought about by the decrease in the interfacial resistances between the gel electrolyte and the counter electrode. In addition, the increase in the J_sc was explained by increases in the apparent diffusion coefficient of I⁻/I₃⁻ when the chain length increased. Decreases in V_oc and ff were explained by back-electron transfers from TiO₂ to iodine in the electrolytes. V_oc of the cells solidified by alkyldiiodide was lower than that solidified by alkyldichloride or alkyldibromide. It was explained by negatively shifted redox potential of I⁻/I₃⁻, compared with those for Cl⁻/Cl₂ or Br⁻/Br₂.     

Carbon nanosheets as the electrode material in supercapacitors

Carbon nanosheets are comprised of 1–7 graphene layers that are predominantly vertically oriented with respect to a substrate. The thickness and morphology of the nanosheets can vary depending on the growth precursor and the substrate temperature. They have an ultra-low in-plane resistivity. The capacitance of carbon nanosheets was measured by cyclic voltammetry in a standard electrochemical three-electrode cell, which contains a platinum counter electrode and a standard mercury/mercurous sulfate reference electrode in 6 M H₂SO₄ electrolyte. As a working electrode, the capacitance of carbon nanosheets per area was found to be 0.076 F cm⁻². A mathematical model was used to simulate the total possible capacitance of a virtual supercapacitor cell that contains carbon nanosheets as the electrode material and found to be 1.49 × 10⁴ F.     

A novel bifunctional electrocatalyst for unitized regenerative fuel cell

5 wt.% of platinum (Pt) nanoparticles are highly dispersed on the surface of IrO₂ by chemical reduction, and the catalyst is mixed with Pt black to be used as a novel bifunctional oxygen electrocatalyst for the unitized regenerative fuel cell (URFC). The novel cell has been evaluated in the hydrogen and oxygen fuel cell and water electrolysis modes, and compared to a similar cell with an oxygen electrode using conventional mixed Pt black and IrO₂ catalyst. With the novel oxygen electrode catalyst, the highest fuel cell power density is 1160 mW cm⁻² at 2600 mA cm⁻²; the overall performance is close to that with the commercial Pt supported on carbon catalyst and about 1.8 times higher than that with the conventional mixed Pt black and IrO₂ catalyst. Additionally, the cell performance for water electrolysis is also slightly improved, which is probably the result of lower interparticle catalyst resistance with 5 wt.% Pt on IrO₂ compared to no Pt on IrO₂.     

Electrochemical impedance studies of electrooxidation of methanol and formic acid on Pt/C catalyst in acid medium

Cyclic voltammetry (CV), amperometric i − t experiments, and electrochemical impedance spectroscopy (EIS) measurements were carried out by using glassy carbon disk electrode covered with the Pt/C catalyst powder in solutions of 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ HCOOH at 25 °C, respectively. Electrochemical measurements show that the activity of Pt/C for formic acid electrooxidation is prominently higher than for methanol electrooxidation. EIS information also discloses that the electrooxidation of methanol and formic acid on the Pt/C catalyst at various polarization potentials show different impedance behaviors. The mechanisms and the rate-determining steps of formic acid electrooxidation are also changed with the increase of the potential. Simultaneously, the effects of the electrode potentials on the impedance patterns were revealed.     

Photocalorimetry at semiconductor electrodes: theory, technique and applications

A review is given on the theory and technique of photocalorimetry using photo-acoustic or pyroelectric detection of the temperature change at the back surface of a thermally thin semiconductor electrode under chopped monochromatic illumination. Relations are given for determining the internal quantum efficiency (η_a) of the photocurrent, the upper limit of the internal energy conversion efficiency (L_G) attainable with the electrode in a photoelectrolytic or regenerative cell, and the Peltier heat (Q_PE) for the reaction at an n-type photoanode. The theory includes competitive processes such as photocorrosion of the semiconductor and oxidation of a solute. Photoanodic oxidation of water has been studied on polycrystalline n-TiO₂ thin-film electrodes (rutile and Be-doped anatase) at pH 0.3–13.8. The lower values of η_a and L_G at the anatase film are attributed to a shorter hole diffusion length than in the rutile film. The pH dependence of Q_PE is similar at both modifications of TiO₂. It is controlled mainly by the pH-dependent entropy change of the leading net reactions in acid and strongly basic solution. The faradaic efficiency for photoanodic oxidation of Cl⁻ competing with that of water at the rutile electrode has been determined. The values for formation of Cl₂ in acid solution decrease with increasing pH. Results for pH 9 are consistent with formation of ClO⁻. Photocorrosion of n-GaAs (100) in 0.5 M H₂SO₄ has been studied as a function of concentration of I⁻. On increasing [I⁻], the faradaic efficiency for photoanodic oxidation of I⁻ approaches unity at [I⁻] ≥ 4 M, indicating complete stabilization of GaAs. For n-GaAs/I⁻ (7 M), L_G was 5% at 633 nm. At a photoanode made from natural pyrite, L_G was circa 0.5% for I⁻ (1 M)/I₂ (1 mM) in 0.5 M H₂SO₄ at 633 nm.     

Enhanced catalytic properties from platinum nanodots covered carbon nanotubes for proton-exchange membrane fuel cells

An efficient fabrication method for carbon nanotube (CNT)-based electrode with a nanosized Pt catalyst is developed for high efficiency proton-exchange membrane fuel cells (PEMFC). The integrated Pt/CNT layer is prepared by in situ growth of a CNT layer on carbon paper and subsequent direct sputter-deposition of the Pt catalyst. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrate that this Pt/CNT layer consists of a highly porous CNT layer covered by well-dispersed Pt nanodots with a narrow size distribution. Compared with conventional gas-diffusion layer assisted electrodes, the CNT-based electrode with a Pt/CNT layer acting as a combined gas-diffusion layer and catalyst layer shows pronounced improvement in polarization tests. A high maximum power density of 595 mW cm⁻² is observed for a low Pt loading of 0.04 mg cm⁻² at the cathode.     

Incorporation of polyaniline into macropores of three-dimensionally ordered macroporous carbon electrode for electrochemical capacitors

Three-dimensionally ordered macroporous (3DOM) carbons having walls composed of mesosized spherical pores were prepared by a colloidal crystal templating method. A composite electrode consisting of bimodal porous carbon and polyaniline (PAn) was prepared by electropolymerization of aniline within the macropores of the bimodal porous carbon. It was found that the deposition of PAn decreased the porosity and specific surface area (SSA) of the electrode. The electrochemical properties of the composite electrode were characterized in a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 1 mol dm⁻³ LiPF₆. The discharge capacity of the carbon–PAn composite electrode was 111 mAh g_carbon–PAn⁻¹ in the potential range of 2.0–4.0 V vs. Li/Li⁺, which corresponded to a volumetric discharge capacity of 53 mAh cm⁻³. Both the double-layer capacity (30 mAh g⁻¹) and the redox capacity of PAn (81 mAh g⁻¹) contributed to the discharge capacity of the composite electrode. The carbon–PAn composite showed good rate capability, and the discharge capacity at a high current density of 6.0 A g⁻¹ was as high as 81 mAh g⁻¹.     

Electrospun porous Fe2O3 nanotubes as counter electrodes for dye‐sensitized solar cells

Porous Fe₂O₃ nanostructures were synthesized through electrospinning of Fe (NO₃)₃/polyvinylpyrrolidone followed by calcination in air. The morphology of the resultant Fe₂O₃ was tuned by changing the ratio between Fe (NO₃)₃ and the polymer matrix. The performance of these nanostructures as counter electrodes in dye‐sensitized solar cells (DSSCs) was investigated. It was found that nanotubes exhibit significantly higher catalytic efficiency toward reducing I^?/I₃^? electrolytes than nanorods and nanobelts, showing a photoelectric conversion efficiency of 4.0%, also superior to a range of transition metal oxides. Furthermore, the nanotube‐based counter electrode showed lower resistance than other Fe₂O₃ nanostructures. These results were attributed to the high specific surface area (90.2 m² g^?1) of the nanotubes, which provides a large reaction site and can promote the charge transfer at the electrode/electrolyte interfaces. The low cost and ease of mass production make Fe₂O₃ nanotube a promising candidate to replace Pt as the counter electrode in DSSCs.     

Donor–acceptor interaction between non-aqueous solvents and I2 to generate I3, and its implication in dye sensitized solar cells

The spectrophotometric properties of I⁻, I₂ and the I⁻/I₂ mixture were studied in 1,2-dichloroethane (DCE), acetone (AC), acetonitrile (ACN), ethanol (EtOH), methanol (MeOH), tertiary-butanol (t-BuOH), dimethylformamide (DMF), propylenecarbonate (PC), 3-methoxypropionitrile (MePN), dimethylsulfoxide (DMSO), dioxane (DIO) and pyridine (PY) solutions. From the investigation it has been realized that in DCE, I⁻, I₂ and I⁻/I₂ mixture have the same absorption peak at 500 nm. I⁻ gives rise to the absorption spectra at about 220, 290 and 360 nm in t-BuOH and in PY solutions. However, in all other solvents the I⁻ generates peaks only around 220 nm. Similarly I₂ and the I⁻/I₂ mixture in all solvents except DCE have indicated similar absorption peaks around 220, 290 and 360 nm. On the other hand, except in PC and DMF, I₂ shows the additional peaks in the range of 380–500 nm which are assigned to the formation of a I₂–solvent complex. The peaks around 290 and 360 nm indicate the presence of I⁻₃ and around 220 nm is the peak of I⁻. The spectral shift of the I₂ solutions in the visible region is interesting and is the core of this report. It points to the importance of donor–acceptor interaction between solvents and iodine. The data obtained in these solvents were well correlated to the donor number (DN) of the solvents. From this correlation the DN of MePN was estimated to 14.6. The absorption peak of I₂ in DCE(DN=0.0) is 500 nm and in PY(DN=33.1) is 378 nm. This peak shift due to solvent effects corresponds to an energy difference close to 0.8 eV. The absorption peak shift due to addition of the 0.0080 vol%. PY(1 mM) in 1 mM I₂-ACN solutions corresponds to ca. 0.6 eV. The blue shift of I₂ absorption in basic solvents indicates the tendency to form a complex. The increase of the efficiency of the dye-sensitized solar cell by addition of PY to I⁻/I⁻₃ ACN solution is suggested to be due to the formation of the dipyridine complex, PY₂I⁺. Such complex formation decreases the amount of I₂ which is expected to be an electron scavenger. We also propose that the more bulky complex, PY₂I⁺ has a slower kinetics with the conduction band electrons, and thus decrease the losses of photocurrent and photopotentials in the solar cell.     

Effect of electrolytes on the photovoltaic performance of a hybrid dye sensitized ZnO solar cell

The efficiency of dye sensitized solar cell depends on the number of factors such as impedance due to anions in the electrolytes, oxidation–reduction process of anions and size of cations of the electrolyte. This paper reports the effect of electrolytes on the photovoltaic performance of hybrid dye sensitized ZnO solar cells based on Eosin Y dye. The size of the cations has been varied by choosing different electrolytes such as LiBr+Br₂, LiI+I₂, tetrapropylammonium iodide +I₂ in mixed solvent of acetronitrile and ethylene carbonate. The impedance of anions has been determined by electrochemical impedance spectra. It is observed that Br⁻/Br₃⁻ offers high impedance as compared to I⁻/I₃⁻ couple. The oxidation–reduction reactions of electrolytes are measured by linear sweep voltammogram. It is found that Br⁻/Br₃⁻ is more suitable than an I⁻/I₃⁻ couple in dye sensitized solar cell (DSSC) in terms of higher open-circuit photovoltage production and higher overall energy conversion efficiency. This is attributed to more positive potential of the dye sensitizer than that of Br⁻/Br₃⁻. The gain in V_oc was due to the enlarged energy level difference between the redox potential of the electrolyte and the Fermi level (E_f) of ZnO and the suppressed charge recombination as well.     

On the use of triethylamine hydroiodide as a supporting electrolyte in dye-sensitized solar cells

For the first time, the application of a molten salt, triethylamine hydroiodide (THI), as a supporting electrolyte was investigated for the dye-sensitized solar cells (DSSCs). Titanium dioxide (TiO₂) electrode was modified by incorporation of high- and low-molecular weight poly(ethylene glycol) along with TiO₂ nanoparticles of two different sizes (300 nm (30 wt%) and 20 nm (70 wt%)). The highest apparent diffusion coefficient (D) of 8.12×10⁻⁶ cm² s⁻¹ was obtained for I⁻ (0.5 M of THI) from linear sweep voltammetry (LSV). Short-circuit current density (J_sc) increases with the concentration of THI whereas open-circuit potential (V_oc) remains the same. Optimum J_sc (19.28 mA cm⁻²) and V_oc (0.7 V) with a highest conversion efficiency (η) of 8.45% were obtained for the DSSC containing 0.5 M of THI/0.05 M I₂/0.5 M TBP in CH₃CN. It is also observed that the J_sc and η of the DSSC mainly relates with the D values of I⁻ and charge-transfer resistances such as R_ct1 and R_ct2 operating along Pt/TiO₂ electrolyte interface, obtained from LSV and electrochemical impedance spectroscopy (EIS). For comparison, tetraethylammonium iodide (TEAI) and LiI were also selected as supporting electrolytes. Though both the THI and TEAI have similar structures, replacement of one methyl group by hydrogen improves the efficiency of the DSSC containing the former electrolyte. Further, the DSSC containing THI exhibits higher J_sc and η than LiI (7.70%), from which it is concluded that THI may be used as an efficient and alternative candidate to replace LiI in the current research of DSSCs.     

Binary room-temperature ionic liquids based electrolytes solidified with SiO2 nanoparticles for dye-sensitized solar cells

In this study, binary ionic liquids (bi-IL) of imidazolium salts containing cations with different carbon side chain lengths (C = 2, 4, 6, 8) and anions such as iodide (I⁻), tetrafluoroborate (BF₄⁻), hexafluorophosphate (PF₆⁻) and trifluoromethansulfonate (SO₃CF₃⁻) were used as electrolytes in dye-sensitized solar cells (DSSCs). On increasing the side chain length of imidazolinium salts, the diffusion coefficients of I₃⁻ and the cell conversion efficiencies decreased; however, the electron lifetimes in TiO₂ electrode increased. As for different anions, the cell which contains 1-butyl-3-methyl imidazolium trifluoromethansulfonate (BMISO₃CF₃) electrolyte has better performance than those containing BMIBF₄ and BMIPF₆. From the impedance measurement, the cell containing BMISO₃CF₃ electrolyte has a small charge transfer resistance (R_ct2) at the TiO₂/dye/electrolyte interface. Moreover, the characteristic frequency peak for TiO₂ in the cell based on BMISO₃CF₃ is less than that of BMIBF₄ and BMIPF₆, indicating the cell with bi-IL electrolyte based on BMISO₃CF₃ has higher electron lifetime in TiO₂ electrode. Finally, the solid-state composite was introduced to form solid-state electrolytes for highly efficient DSSCs with a conversion efficiency of 4.83% under illumination of 100 mW cm⁻². The long-term stability of DSSCs with a solidified bi-IL electrolyte containing SiO₂ nanoparticles, which is superior to that of a bi-IL electrolyte alone, was also presented.     

Influence of alkylaminopyridine additives in electrolytes on dye-sensitized solar cell performance

The influence of alkylaminopyridine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell with an I⁻/I₃⁻ redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 20 different alkylaminopyridines under AM 1.5 (100 mW/cm²). The alkylaminopyridine additives tested had varying effects on the performance of the cell. All the additives decreased the short circuit photocurrent density (J_sc), but increased the open-circuit photovoltage (V_oc) of the solar cell. Molecular orbital calculations imply that the dipole moment of the alkylaminopyridine molecules influences the J_sc of the cell and that the size, solvent accessible surface area, and ionization energy all affect the V_oc of the cell. The highest V_oc of 0.88 V was observed in an electrolyte containing 4-pyrrolidinopyridine, which is comparable to the maximum V_oc of 0.9 V for a cell consisting of TiO₂ electrode and I⁻/I₃⁻ redox system.     

High-performance and low platinum loading Pt/Carbon black counter electrode for dye-sensitized solar cells

Pt/Carbon black counter electrode for dye-sensitized solar cells (DSSCs) was prepared by reducing H₂PtCl₆ with NaBH₄ in carbon black. The Pt/Carbon black electrode had a high electrocatalytic activity for iodide/triiodide redox reaction. Using the Pt/Carbon black counter electrode, DSSC achieved 6.72% energy conversion efficiency under one sun illumination. Pt/Carbon black electrode shows the same energy conversion efficiency and lower cost compared with Pt electrode, which makes it available in DSSCs practical applications.     

Production of large-area polymer solar cells by industrial silk screen printing, lifetime considerations and lamination with polyethyleneterephthalate

The possibility of making large area (100 cm²) polymer solar cells based on the conjugated polymer poly 1,4-(2-methoxy-5-ethylhexyloxy)phenylenevinylene (MEH-PPV) was demonstrated. Devices were prepared by etching an electrode pattern on ITO covered polyethyleneterephthalate (PET) substrates. A pattern of conducting silver epoxy allowing for electrical contacts to the device was silk screen printed and hardened. Subsequently a pattern of MEH-PPV was silk screen printed in registry with the ITO electrode pattern on top of the substrate. Final evaporation of an aluminum electrode or sublimation of a Buckminsterfullerene (C₆₀) layer followed by an aluminum electrode completed the device. The typical efficiency of the prototype devices consisting of three solar cells in series were 0.0046% (under AM1.5 conditions) with open-circuit voltages (V_oc) of 0.73 V and short-circuit currents (I_sc) of 20 μA cm⁻². The half-life based on I_sc in air for the devices were 63 h. The cells were laminated in a 125 μm PET encasement. Lamination had a negative effect on the lifetime.We demonstrate the feasibility of industrial production of large area solar cells (1 m²) by silk screen printing and envisage the possibility of production volumes 10000 m² h⁻¹ at a cost that is on the order of 100 times lower than that of the established monocrystalline silicon solar cells in terms of materials cost.     

A photovoltaic cell incorporating a dye-sensitized ZnS/ZnO composite thin film and a hole-injecting PEDOT layer

A photovoltaic cell containing a dye-sensitized ZnS/ZnO composite thin film was studied. ZnS was thermally evaporated or electrodeposited onto conducting fluorine-doped tin oxide glass; then a particulate ZnO layer was pasted and sintered to form a ZnS/ZnO composite layer. A visible light source was utilized to excite the Ru-dye, which was adsorbed onto the surface of the ZnO. The ZnS layer is believed to provide an alternative pathway for electrons to move across ZnO barriers. This alternative pathway with the composite layer structure provides higher power efficiency than does a single layer of ZnO or ZnS. A hole-injecting, p-type poly(3,4-ethylenedioxythiophene) (PEDOT) thin film was also introduced to substitute for the Pt catalytic layer which helps with the rejuvenation of I⁻ ions. Although the p-type semiconductor behavior increased the open circuit voltage (V_oc), the power efficiency decreased because the I⁻ rejuvenation rate was much slower on PEDOT than on Pt.     

A photogalvanic cell utilizing the photodissociation of iodine in solution

A photogalvanic cell is reported in which visible light is converted into electricity via the photodis-sociation of iodine in aqueous and non-aqueous solution. A mechanism for the aqueous system probably involves the formation of I₂⁻ or I radical which becomes reduced at the transparent, illuminated electrode. The efficiency is very sensitive to formal concentration of I₂, and wavelength, and somewhat less on electrode material, with the highest efficiency observed (0.03 per cent) in a cell containing indium-tin oxide electrode at 4047 Å and 9F I₂, 3F NaI in acetonitrile.